It is customary in fiber optic cable to configure the basic elements forming the cable in a concentric and symmetrical pattern. This usually means that a center core means (may be a strength member) is provided as the initial building block. About the core's periphery, circumferentially and symmetrically are disposed a plurality of tubes of predetermined diameter and in each tube there are one or more optical fibers. In most instances, the tubes contain a filling material usually of a hydrocarbon composition. Depending on the core size (diameter), only a given number of tubes of given size (equal or unequal to the diameter of the core) can be concentrically and symmetrically disposed on the core's periphery.
Fundamental to any fiber optic configuration is the number of fibers assigned to the cable (fiber count) and the number of fibers allocated to each tube. There are many instances where the fiber count is distributed over "X" number of tubes, but one would have to use "Y" number of tubes disposed around the periphery of the core in order to achieve a symmetrical and concentric cable configuration, i.e., "Y" is greater than "X". In such cases, the known prior art practice is to use tubes in which there are no fibers ("dummy tubes") or a solid rod of some suitable material (preferably the same material out of which the tubes are made) to fill out the required circumscribing tubes so that symmetrical concentricity is achieved. Prior art practice, because of concerns arising out of the behavior of dissimilar material under certain conditions (thermal expansion and contraction for example), have followed the rule that dummy tubes or rods should be of the same material as the tubes carrying the optical fibers. Polyamid or a fluropolymer type plastic is an example of the material generally employed to make the tubes containing the optical fibers, the dummy tubes and rods. Compared to other types of plastic material such material is expensive. Thus, it is desired to design a structure of a dummy tube or rod, made from cheaper materials, that would have no adverse effect on the tubes containing optical fibers, the optical fibers themselves and the overall function of a cable made using same when a cable employing such design is subjected to wide thermal swings (-40.degree. to 70.degree. C.). It is to the solution of this problem that this invention is directed.